Modern drilling techniques employ various means of steering a well when exploring for oil and/or gas. One issue is to ensure that the well's directional profile follows plan in a reasonably accurate manner. This requires steering techniques that make use of downhole drilling motors that are able to steer in certain directions in a controllable manner, requiring the directional driller at surface to have information pertinent to the direction the well is currently taking; this is provided by one of several telemetry techniques. One technique is to produce timed pressure pulses in the drilling fluid (mud) that are detectable at surface. The periodicity of these pulses encodes data typically associated with the orientation of a downhole sensor and certain other information related to downhole conditions, thereby enabling the driller to safely steer the well from the surface. In general this and similar methods of encoding data and sending such information to the surface are called ‘Measurement While Drilling’ (MWD).
There are several methods by which one can produce the pressure pulses—one common approach is to rotate a rotary vane relative to a stationary vane to vary the degree of obstruction to downhole mud flow.
The downhole fluid pressures can be 20,000 psi or greater. A spindle turns the aforementioned rotary vane and is usually driven by an electric motor, powered by a battery and controlled by electronic circuits. It is necessary to protect the motor and associated devices from the mud; this is conventionally accomplished by incorporating seals between the spindle and the spindle housing. Because the pressure difference between external and internal pressure can be as great as 20,000 psi, a significantly robust seal is needed. This can lead to seal and/or spindle wear problems because of the friction such a seal engenders. Furthermore, the power necessary to rotate the spindle would be dominated by the frictional force between seal and spindle, and as the usual power source to run the motor is a primary cell lithium battery this can become very expensive and/or lead to a greatly reduced downhole operation time. In order to minimize these effects it is advantageous to run the driven section of the spindle behind the seal and the electric motor in oil while balancing the oil pressure with the mud pressure, leaving only a small net pressure differential across the seal. The housing containing the motor assembly can be pressurized to approximately the same as the external pressure by a simple compensation device, such a rubber sleeve internal to the housing, forming a flexible barrier between mud and oil. This enables a relatively small seal to be used, typically incorporating elastomeric seal lips or materials such as PTFE that reduce the coefficient of friction. Despite this property most seals have to cope with wear aggravated by abrasive particulate matter contained in the drilling fluid. Although means can be used to reduce this (for example see Hatch et al., U.S. Pat. No. 7,055,828 whereby the seal incorporates labyrinthine lips in order to help exclude debris) wear can enlarge the opening, allowing drilling fluid ingress and hence lead to ultimate failure or increased friction due to loss of appropriate lubrication, this effect potentially inducing skew distortion that further enables more contamination (see for example Conroy et al., U.S. Pat. No. 6,315,302 in an attempt to reduce the problem by the use of a specifically shaped and energized seal).
Despite these advances, seal wear is inevitable, particularly when the seal has to exclude particulate-containing drilling fluid, so the best that can be done at present is to ensure that the spindle surface minimizes entraining fine particulate abrasive matter that, in effect, will act like rubbing sandpaper along the soft seal lips. Thus it is desirable to make the spindle surface as smooth as is possible, consistent with other material strength properties necessary to support a pressure pulse-producing downhole valve mechanism, and also itself have very low wear properties. The present industry practice is to apply a thermal spray coating of a hard coating, (typically tungsten carbide) to improve the wear properties of the bare metal (typically a stainless steel such as 17-4). An exemplary such spindle is shown in FIG. 1 (Prior Art) wherein a thin tungsten carbide coating 31 of 0.005″ to 0.010″ has been applied to the base spindle material. Adequate adhesion of tungsten carbide to the metal spindle requires the addition of a binding material such as cobalt or nickel, thereby forming a metallic matrix that requires subsequent grinding to an appropriate smoothness, typically 2 to 8 μ-inches. Of the two binding materials, nickel is generally preferred for its increased resistance to corrosion in the presence of drilling fluids. Regardless of the choice of binder material, the performance of the coating is limited to the porosity, adhesion and corrosion resistance of the available compounds that are suitable for application by thermal spray. Other undesirable issues with hard coatings include a reduction of mechanical properties of the base metal (usually because the necessary heating required to bond the applied coating to the base and sometimes the undesirable formation of intermetallic compounds), limited coating depth that is susceptible to degradation or delamination, no in-situ or in-house repair ability with the accompanying high cost of outsourcing the coating removal and re-application, and inferior performance of the coating relative to a similar solid part made from powder metallurgy.
Maintenance of the surface quality is a major issue in the long term performance of the sealing surface. Degradation of the seal surface occurs frequently and plays a large role in limiting the lifetime of the spindle and seal. Also, refinishing or replacement of the sealing surface is routinely required—a discussion of such concerns can be found in the Kalsi Seals Handbook, 2005, Section 3, pp 12-15.
The underlying spindle material is usually chosen to be a tough, corrosion-resistant metal. In a MWD drilling environment, excellent mechanical properties are required for this metal to resist the axial, rotational and radial forces on the rotary vane due to drill string shock (thousands of Gs) and vibration (tens of Gs), as well as high frequency vibration and large pressure loading due to the generation of ˜300 psi pressure pulses in turbulent flow. In addition, the most highly stressed area of the spindle is exposed to the drilling fluid and must survive chemical attack and erosional flow created when pressure pulses are produced.
An alternative method of producing a pressure pulse is to linearly move a poppet into and out of an orifice (poppet & orifice valve), thereby changing the drilling fluid flow rate through the valve and using this to subsequently create pressure pulses. At issue here is the need to drive the shaft, the distal end of which is typically within the drilling fluid. This necessitates a seal because the shaft driver is usually an electric motor or a solenoid, both of which normally require protection within a lubricant-filled housing. Such seals are either similar to rotary seals or are of a bellows type, and the technique similarly needs pressure compensation. The method is typified by U.S. Pat. No. 6,898,150 (Hahn et al., 2005) that utilizes a stationary valve and reciprocating poppet driven by an electric motor in a lubricant-filled housing. The problems inherent in this approach are either that the seal has enhanced wear compared to a rotary seal due to the ingress of contaminants between the shaft and the elastomeric seal lips, particularly as the spindle is pulled back through the seal, or there is restricted stroke because the seal is a bellows type.
The properties of the spindle thus far described should preferably incorporate a smooth sealing surface in the location of the elastomeric seal, be resistant to erosion and chemical attack, and also be mechanically strong enough to withstand severe downhole axial and radial forces while supporting and operating the valve, and be slender enough to minimize the net frictional (circumferential) forces as the spindle moves within the seal. Other desirable properties are to make this spindle inexpensive to manufacture and easy to maintain. The present art does not optimally address the best economically-possible smoothness requirement although a step toward better maintenance was taken by Messenger (U.S. Pat. No. 4,208,057) in his teaching of a 2-part spindle, whereby the distal end of this design could be removed without undue disturbance of the proximate end for maintenance purposes. But Messenger still falls short in providing both cost effective manufacture and a superior smooth sealing surface. Further, no other art to date addresses these issues plus methods of alleviating other concerns noted herein, within one comprehensive design.